1. Field of the Invention
The subject invention relates generally to digital data communications and more particularly to an apparatus employing multiplexing to combine a plurality of digital data channels for transmission over a single transmission path. The invention provides an apparatus for automatically varying the channel configuration and is particularly useful in conjunction with digital data modems.
2. Background of the Invention
Digital data modems are commonly used for interfacing with data processing apparatus at either end of a transmission channel such as a telephone line. Multi-port modems are also known which provide a plurality of channels for communication with a similar plurality of channels of the associated data processing apparatus. The multiple channel information is multiplexed for transmission over an individual line. Multiplexing is typically accomplished by TDM (time division multiplexing) with bit-by-bit or word-by-word interlacing of data from each data port to a single aggregate channel. Of course, many methods of multiplexing are known and could possibly be used according to the invention.
In initiating communication across a transmission line between modems, generally a sequence of initializing signals are generated in what is sometimes referred to as a "handshaking" operation. Such signals may indicate when a data source actually wants to transmit data or wants a channel at its disposal ready for transmission. An example of the former alternative currently in use is RTS (request to send or ready for sending). An example of the second alternative is DTR (data terminal ready). RTS is present only while a transmission is in progress, while DTR will be present throughout the time that the data source is engaged in interactive communication with a device such as a CPU. It is also known that DCD (data carrier detect) is not only provided in the presence of a data carrier from a communicating modem, but it is also set high or low upon receipt of a coded RTS signal.
Known multi-port modems contain the necessary circuitry for switching between various port configurations in response to commands set manually by an operator. Such channel allocation or port reconfiguration is useful when data traffic patterns differ relatively infrequently in a known manner. It is then possible to set up a mode switching schedule which requires the intervention of an operator from time to time to effect mode changes. Moreover the actions of two operators, one manually operating a modem at each end of the transmission line, must generally be coordinated. To make more efficient use of the expensive telephone channels, it is desirable to have a dynamic port reconfiguration capability wherein the data processing apparatus and modem system would cooperate to reconfigure ports rapidly and automatically without operator intervention.
U.S. Pat. No. 4,270,202 to E. B. Studdard et al., and assigned to the assignee of the present invention, describes circuitry for automatically and dynamically reconfiguring the channel or port configuration of a modem handling a plurality of ports. In this patent, standard signals such as DTR (data terminal ready) are used to provide a code to a master modem indicating the particular port configuration requested by a cooperating apparatus. The requested configuration code is continuously compared to a code stored by a shift register controlling the actual modem port configuration. When the requested code changes, an indication of the code change is transmitted to a communicating slave modem. A control signal is sent, after a suitable delay, to the shift register to conform its contents to the newly requested configuration code. During the delay, the slave modem switches to the newly requested configuration. The circuitry follows a defined sequence of configuration switching, dropping channels successively and adding channels by passing through the state where all channels are active. In an alternative system operation, each of two communicating modems may be synchronized by the cooperating apparatus such that a configuration change need not be communicated across the transmission line.
The Omnimode.RTM. series of modems manufactured by Racal-Milgo division of Racal Data Communications, Inc., the assignee of the present invention, uses a secondary channel (out of band) for dynamic port allocation of the bandwidth of a port when it becomes idle or to reallocate the same when the idle port becomes active. In this system the master unit and the remote unit are not synchronized in implementing the port reconfiguration; consequently, data errors are introduced upon such change.
In the C.C.I.T.T.V.33 recommendation, an interruptive retraining is used to transmit modem speed and one of a limited number of possible multiplexer configuration when configuration change is desired. This requires interruption of the aggregate channel, and thus unaffected ports, for several seconds.
U.S. Pat. No. 4,622,685 to Dhawan et al., and also assigned to the assignee of the present invention, describes an RTS/DCD simulator for use in a data modem to simulate these signals by transmission of predetermined codes. This technique has been used in both scrambled and unscrambled form commercially. RTS and DCD simulation for modem communication has also been described and specified by the C.C.I.T.T. modem specification V.33.